Formation of large arrays of zinc oxide nanostructures using electrodeposition

Abstract

In an aqueous solution of a zinc salt, counter, reference and working electrodes are placed, and an electric potential is applied across the working and reference electrodes. A gas including oxygen and an inert gas is introduced into the aqueous solution. Responsive to these conditions an array of zinc oxide nanostructures grows on a conductive nucleation plate that is a part of the working electrode. The nanostructures have sharp tips, have a more efficient electron emissivity than nanorods made from other materials, and can be used in fabricating field emission lamps and displays.

Description

BACKGROUND OF THE INVENTION[0001]Much present-day scientific research involving information technology, display devices and the materials to make such devices is focused on nanotechnology in order to further advance miniaturization. One approach is from the bottom up, and concerns the synthesis of nanostructures which have utility in the semiconductor art. Zero-dimensional quantum dots, one-dimensional quantum wires, nanowires and nanorods have been widely suggested.[0002]International research efforts to synthesize nanostructures have included semiconductors such as silicon, germanium, Al—Ga—In—P—N systems, ZnO, SnO2 and SiC. In particular, the oxide semiconductor ZnO has a wurtzite structure with direct band-gap energy of 3.37 eV and a very high excition binding energy of 60 meV at room temperature. If problems concerning production can be overcome, ZnO based materials could replace conventional materials in areas which require high emission characteristics.[0003]Field emission di...

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Benefits of technology

[0005]Preferably, the zinc salt is one with a high solubility in water and more preferably is selected from the group consisting of ZnNO3 and ZnCl2. At the beginning of the process, the zinc salt should be present in a concentration of about 0.0001 M to about 0.03 M. Preferably, the concentration of zinc in the aqueous solution is maintained at or above a predetermined minimum, all through a first growth period. If this is done, during this first period the nanostructures will grow as prismatic rods or structures, of substantially constant areal cross-section. During a second period, which can merely be a continuation of the first period under the same reaction conditions, the concentration of zinc drops, as by exhausting itself by being electrodeposited on the free ends of the nanorods or structures. As the concentration of zinc drops below the predetermined minimum, the areal cross section of a second length of the structure will begin to shrink. In this way, a sharp tip on the end of a nanotower can be achieved.

[0008]According to another aspect of the invention, the above process produces an array of zinc oxide nanostructures on a conductive nucleation surface. Each nanostructure has a base on the order of 200 nanometers to one micrometer in diameter and has a tip, which is spaced from a tip of a next adjacent nanostructure by about 250 nanometers to one micrometer. A free end of each nanostructure is pointed, enhancing the utility of the nanostructure as an efficient electron emitter. Preferably, each nanostructure is a hexagonal nanotower wherein, throughout a first length starting from its base on the nucleation surface, the nanotower has a substantially uniform cross-sectional area. A second length of the nanotower, spaced from the nucleation surface by the first length, has a cross-sectional area which decreases as a function of the distance from the nucleation surface, and preferably terminates in a point. More preferably the first length of the nanotower substantially takes the form of a hexagonal prism and the second length substantially takes the form of a hexapyramid. Each nanotower is monocrystalline, permitting its operation as a semiconductor-based electron emitter for field emission applications.

[0009]As deposited on a transparent substrate such as glass, the hexagonal nanotower array of the invention has particular utility as an electronic emitter component of a field emission display or lamp. Very large area displays can be formed using this invention, as there are no constraints imposed by the pressure under which the process is conducted (it can be atmospheric rather than be done in a vacuum) or temperature. The produced field emission display requires no manual patterning and no metallization of the individual emitters. Each nanotower has a high aspect ratio.

Problems solved by technology

But the feasibility of mass-producing FEDs for the consumer market has yet to be demonstrated.

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